The present invention relates to a bias circuit, and particularly to a bias circuit capable of outputting a bias current that depends on current amplification gains β of transistors and resistance values without depending on the accuracy of a required reference voltage.
A conventional bias circuit comprises an NMOS transistor for constant current source (hereinafter called “a constant current source transistor”) 2 whose constant current value is controlled based on the output of a feedback amplifier, a differential pair of NMOS transistors (hereinafter called “differential transistors”) equal in dimension from each other, which is connected to the drain of the constant current source transistor, diode-connected load PMOS transistors (hereinafter called “load transistors”) respectively connected to the drains of the differential transistors, current mirror PMOS transistors (hereinafter called “mirror transistors”) which constitute the load transistor and a current mirror, and a resistor 9 which receives a current that flows through the mirror transistor. The drain side of the mirror transistor serves as a current output terminal.
Different reference voltages are respectively applied to the gates of the differential transistors, and another reference voltage is further applied to a positive input terminal of the feedback amplifier.
When, for example, the differential transistors are increased in current amplification gain β due to variations in manufacture in such a bias circuit, the feedback amplifier controls a current flowing through the constant current source transistor so as to be reduced. Accordingly, the mirror transistor outputs an output current that depends on the current amplification gains β of the differential transistors and the resistance value of the resistor.
The conventional bias circuit is accompanied by a problem that the accuracy of the reference voltages applied to the differential transistors is degraded, and when an input potential difference varies, the output current also varies eventually.